The following meanings for the abbreviations used in this specification apply:    AMBR Aggregate Maximum Bit Rate    AMC Adaptive modulation & coding    ARP Allocation and Retention Priority    BSR Buffer Status Report    CA Carrier aggregation    CC Component carrier    CoMP Coordinated Multi Point    CRC Cyclic Redundancy Check    DL Downlink    DRB Data Radio Bearer    DRX Discontinuous Reception.    eNB enhanced Node-B    E-UTRA Evolved Universal Terrestrial Radio Access.    GBR Guaranteed Bit Rate    GPRS General Packet Radio Service.    GTP GPRS Tunneling Protocol.    HARQ Hybrid automatic repeat request    HO Handover    HSPA High-Speed Packet Access    ID Identity    I-HSPA Internet High-Speed Packet Access    LCID Logical Channel ID.    LTE Long term evolution    LTE-A LTE-Advanced    MAC Media Access Control    PCell Primary Cell    PDCCH Physical Downlink Control Channel    PDCP Packet Data Convergence Protocol    PDSCH Physical Downlink Shared Channel    PDU Packet Data Unit    PHY Physical Layer    PRACH Physical Random Access Channel    PUCCH Physical Uplink Control Channel    PUSCH Physical Uplink Shared Channel    QCI Quality of Service (QoS) Class Identifier    QoS Quality of Service    RA Random Access    RLC Radio Link Control    RRC Radio Resource Control    SCell Secondary Cell    TA Time Alignment    TDM Time Division Multiplexing    TEID Tunneling End Identity    UE User equipment    UL Uplink
Embodiments of the present invention relate to a heterogeneous network scenario in which different types of base station nodes are provided. As an example, a case with macro-cells and pico-cells is considered, although the small cells could be microcells or even Femtocells in future standardization releases as well. That is, a case is considered in which within the coverage area of a large cell other smaller cells are provided. Hence, the larger cell is also referred to as umbrella cell. For such cases, the macro-cells could be installed to operate on certain frequencies, while the pico-cells are using other frequencies. In this embodiments, eNB controlling macro-cell is called macro-eNB and eNB controlling pico-cell is called pico-eNB and an X2 interface is defined between the macro-eNB and the pico-eNB.
Such a situation is shown in FIG. 1. In this example, a macro-cell controlled by macro-eNB is using carrier F1, whereas pico-cells controlled by pico-eNB are using carrier F2.
In such a situation an X2-based inter-site LTE carrier aggregation (CA)/COMP can be applied. When configured with inter-site CA/CoMP, a UE is connected to multiple non-collocated eNBs via separate frequency carriers or same frequency. One eNB is controlling a primary cell (PCell) or primary component carrier, and possibly one or more secondary cells (SCell) or secondary component carrier, while the other eNB is controlling one or more SCells or secondary component carriers. Major modifications to the Release 10 specifications are needed to support this type of CA configuration.
An example is shown in FIG. 1, in which a UE is connected to a macro-eNB (a first eNB) and to a pico-eNB (a second eNB). The macro-eNB is using carrier F1 (PCell or primary component carrier), whereas the pico-eNB is using carrier F2 (SCell or secondary component carrier). Between the two eNBs, an X2 interface is provided.
Application PCT/EP2011/055409 introduces a general framework for an inter-site LTE CA as outlined above, and suggests that the configuration of a small cell as SCell is performed using RRC. For Rel-10, carrier aggregation is only supported for intra-eNB, hence the eNB knows all the parameters to be configured for the SCell.
However, when the cells to be aggregated are from different eNBs, coordination via X2 would be required to ensure e.g.:                C-RNTI allocated to the UE is not used by other UEs in the SCell (currently common C-RNTI is used for all the aggregated serving cells);        QoS requirements are met without wasting of resources.        
It could be thought of using the inter-eNB HO preparation procedure as part of the purpose of inter-site SCell preparation, but the two procedures are quite different concerning the following aspects. For example, after C-RNTI allocation by the target cell, the C-RNTI is not used by the UE in the source cell anymore, in order to avoid collision problems in the HO case. Furthermore, the QoS parameters delivered via X2 for HO and QoS parameters for inter-site CA case will be different. In case of HO, the source eNB forwards the QoS parameters as received from MME to the target eNB. However, in case of inter-site CA, PCell eNB will calculate new QoS parameters which fits to pico-cell taking into account the X2 delay between macro and pico, scheduling capacity in the macro, etc.
Thus, application of the HO case for the SCell preparation would not be feasible.
Hence, presently there is no sufficient mechanism to surely applying a carrier aggregation with separate eNBs.